Simultaneous detection of multiple spectra of scattered radiation

ABSTRACT

In an example, an apparatus is described that includes a light source, a holographic optical element, a sampling apparatus, and a detector. The light source is configured to emit a beam of excitation light. The holographic optical element is arranged to convert the beam of excitation light into a plurality of beams of excitation light. The sampling apparatus is arranged to project the plurality of beams of excitation light onto a surface outside the apparatus as a two-dimensional pattern of projection points. The sampling apparatus is further arranged to collect scattered radiation emitted by the surface in response to the two-dimensional pattern of projection points. The detector detects a frequency shift in the scattered radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/781,991, filed on Jun. 6, 2018, which is a 371(c) National PhaseApplication of International Application No. PCT/US2016/028096, filedApr. 18, 2016, both of which are herein incorporated by reference intheir entireties.

BACKGROUND

Raman spectroscopy is a spectroscopic technique that can be used toidentify molecules in a sample. The technique relies on Raman(inelastic) scattering of emitted monochromatic light. The emitted lightinteracts with molecular vibrations, phonons, or other excitations inthe sample, which causes the energy of the emitted photons to be shiftedup or shifted down. Information about the vibrational modes in thesample can be inferred from the shift in energy. This information can,in turn, be used to identify the molecules in the sample, sincevibrational information is specific to the chemical bonds and symmetryof molecules.

Although spontaneous Raman spectroscopy is a powerful moleculardetection technique, Raman-scattered signals tend to be very weak. Thesesignals can be enhanced by many orders of magnitude by using speciallypatterned structures that locally enhance the electric field of thelight source and the emitted light. This technique is known assurface-enhanced Raman spectroscopy (SERS). In SERS, sample moleculesare adsorbed on rough metal surfaces and/or by nanostructures. Forinstance, a liquid sample may be deposited onto a silicon or glasssurface having a nanostructured noble metal surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-level block diagram of an example spectrometer of thepresent disclosure;

FIG. 2 is a more detailed block diagram of an example spectrometer ofthe present disclosure;

FIG. 3 illustrates an example two-dimensional pattern of excitationlight that may be produced by the spectrometers of FIGS. 1 and 2 usingthe respective holographic optical elements;

FIG. 4 illustrates an example substrate comprising an array of spatiallyvarying regions that may be used in conjunction with the spectrometersillustrated in FIGS. 1 and 2; and

FIG. 5 illustrates a flowchart of an example method for moleculardetection.

DETAILED DESCRIPTION

The present disclosure broadly describes a spectrometer and associatedmethod for simultaneously detecting multiple Raman spectra usingsurface-enhanced Raman spectroscopy (SERS). The presence of differentanalytes with similar Raman signatures can make it difficult to identifythe molecular content of a sample. Moreover, some molecules may behavedifferently (e.g., exhibit different levels of enhancement or producedifferent scattered signals) when interacting with different SERSsurfaces. Thus, when dealing with a sample of unknown composition, it ishelpful to collect Raman spectra from the sample under many differentconditions in order to improve the likelihood of capturing adistinguishable feature of each molecular component.

Examples of the present disclosure position a holographic opticalelement between a light source and an array of spatially varying SERSsubstrate regions, in order to project a two-dimensional pattern ofexcitation light onto the spatially varying surface. In one particularexample, each projection point of the pattern is incident upon adifferent one of the substrate regions. Each of the substrate regionswill interact differently with different molecules of the sample,thereby producing different spectra (emitted Raman scattering) for thesame sample when exposed to the excitation light. The emitted Ramanscattering is collected, filtered, and dispersed before being deliveredto a detector. The dispersion causes each projection point of thetwo-dimensional pattern of light to appear to the detector as a band oflight, where different horizontal coordinates along the band correspondto different frequencies of light. As a result, a plurality of spectracan be produced simultaneously in a single measurement by thespectrometer (i.e., a single delivery of measured data to the detector).Each of these spectra may, in turn, be compared to a different databasecorresponding to reference measurements for a different substrate regionin order to identify molecules in the sample. Thus, the likelihood ofidentifying the sample's composition is greatly increased withoutincreasing the number of measurements made.

FIG. 1 is a high-level block diagram of an example spectrometer 100 ofthe present disclosure. In one example, the spectrometer 100 is a Ramanspectrometer. In one example, the spectrometer 100 generally includes alight source 102, a sampling apparatus 104, and a detector 106. Inaddition, a holographic optical element 108 is positioned between thelight source 102 and the sampling apparatus 104.

In one example, the light source 102 is a laser diode that emits a beamof excitation light in the visible, near infrared, or near ultravioletrange.

The holographic optical element 108 is positioned to intercept the beamof excitation light and to convert the beam of excitation light into aplurality of beams of excitation light traveling at different angles. Inone example, the holographic optical element comprises a diffractivemask that contains a superposition of diffraction gratings withdifferent spatial frequencies.

The sampling apparatus 104 is configured to project the plurality ofbeams of excitation light onto a sample 110 as a two-dimensional patternof excitation light. An example two-dimensional pattern of excitationlight that may be projected is illustrated in FIG. 3. The samplingapparatus 104 is further configured to collect scattered radiationemitted by the sample 110 in response to the incidence of thetwo-dimensional pattern of excitation light. In one example, thesampling apparatus 104 may comprise a microscope or a fiber optic probe.

The detector 106 is configured to detect a frequency shift in thescattered radiation collected by the sampling apparatus 104. Asdiscussed above, vibrational modes (and, consequently, moleculeidentities) can be detected from upward or downward shifts infrequencies. In one example, the detector comprises a charge-coupleddevice (CCD) detector.

FIG. 2 is a more detailed block diagram of an example spectrometer 200of the present disclosure. The spectrometer 200 may comprise a specificimplementation of the spectrometer 100 illustrated in FIG. 1. Thus, inone example, the spectrometer 200 is a Raman spectrometer. In oneexample, the spectrometer 200 generally includes a light source 202, afirst lens 204, a laser cleanup filter 206, a holographic opticalelement 208, a beam splitter 210, an objective lens 212, a sample portor sample tray 214, a laser blocking filter 216, a diffraction grating218, a folding mirror 220, a focusing lens 222, and a detector 224.

In one example, the light source 202 is a laser diode that emitsexcitation light in the visible, near infrared, or near ultravioletrange. The first lens 204 is positioned directly in the light source'semission path and is further positioned to focus a beam of excitationlight emitted by the light source 202 onto the laser cleanup filter 206.

In one example, the laser cleanup filter 206 is a laser line or narrowbandpass filter. The laser cleanup filter 206 is positioned to removeunwanted energy in the excitation light, such as secondarytransmissions, background plasma, and other artifacts, and to deliverthe “cleaned” beam of excitation light to the holographic opticalelement 208.

In one example, the holographic optical element 208 is a diffractivemask comprising a superposition of diffraction gratings with differentspatial frequencies. The holographic optical element 208 is positionedto convert the cleaned beam of excitation light into a superposition ofbeams of excitation light traveling at different angles and to deliverthese beams of excitation light to the beam splitter 210.

In one example, the beam splitter 210 is a dichroic mirrored prism. Thebeam splitter 210 is positioned to intercept the beams of excitationlight traveling at different angles and to reflect the beams ofexcitation light onto the objective lens 212. The beam splitter 210 maybe oriented at forty-five degrees, in order to account for anapproximately ninety degree angle between the light source 202 and theobjective lens 212.

The objective lens 212 is positioned to receive the beams of excitationlight from the beam splitter 210 and to convert the different angles atwhich the beams of excitation light are traveling to different positionsor locations on the sample port 214. This creates a two-dimensionalpattern of excitation light at the sample port 214 that may be incidentupon the surface of a sample (e.g., a sample adsorbed onto a SERSsubstrate) positioned at the sample port 214. An example two-dimensionalpattern of excitation light that may be projected is illustrated in FIG.3.

The sample port 214 may comprise a spectrometer output path positionedto provide excitation light to a sample, such as a sample adsorbed ontoa SERS substrate. Additionally, the sample port 214 may include a trayor other mechanism configured to support the sample during measurement.As discussed above, the excitation light is incident upon a sample as atwo-dimensional pattern of projection points.

A sample positioned at the sample port 214 will produce emitted Ramanscattering (i.e., inelastic scattered radiation) in response toincidence of the two-dimensional pattern of excitation light. Theobjective lens 212, described above, is further positioned to collectthe emitted Raman scattering and to deliver the emitted Raman scatteringto the beam splitter 210. The beam splitter 210 is further positioned todeliver the emitted Raman scattering to the laser blocking filter 216.

In one example, the laser blocking filter 216 is a longpass edge orlaser rejection filter. The laser blocking filter 216 is positioned toisolate the Raman signal by removing unwanted scattered energy in theemitted Raman scattering, such as elastic scattered radiation at thewavelength corresponding to the laser line (i.e., Rayleigh scattering),and to deliver the isolated Raman signal to the diffraction grating 218.

The diffraction grating 218 is positioned to receive the isolated Ramansignal from the laser blocking filter 216 and to disperse the isolatedRaman signal before delivering the dispersed isolated Raman signal tothe folding mirror 220.

The folding mirror 220 is positioned to receive the dispersed isolatedRaman signal from the diffraction grating 218 and to deliver thedispersed isolated Raman signal to the focusing lens 222.

The focusing lens 222 is positioned to receive the dispersed isolatedRaman signal from the folding mirror 220 and to project the dispersedisolated Raman signal onto the detector 224.

In one example, the detector 224 is a charge-coupled device (CCD)detector. Due to the dispersion of the isolated Raman signal by thediffraction grating, each projection point of the two-dimensionalpattern of excitation light that is incident upon a sample will appearto the detector 224 as a band of light. Different horizontal coordinatesalong the band of light will correspond to different frequencies, fromwhich shifts in frequencies (and, thus, vibrational modes and moleculeidentities) can be detected.

In one example, the orientation angle of the two-dimensional pattern ofexcitation light relative to the diffraction grating 218 can beconfigured to optimize the fraction of the detector area that collectsuseful spectral data. For instance, in one example, this orientationangle is configured at fifteen degrees.

FIG. 3 illustrates an example two-dimensional pattern 300 of excitationlight that may be produced by the spectrometers 100 and 200 of FIGS. 1and 2 using the respective holographic optical elements 108 and 208. Asillustrated, in one example, the two-dimensional pattern comprises arectangular array of projection points (e.g., dots or spots), such as afour-by-four array.

The angle, θ, represents a relative orientation of the two-dimensionalpattern 300 to the diffraction grating 218 of the spectrometer 200. Thisangle can be configured to optimize fraction of the detector area thatcollects useful spectral data, as discussed above. In one example, thisangle is approximately fifteen degrees.

FIG. 4 illustrates an example substrate 400 comprising an array ofspatially varying regions A1-D4 that may be used in conjunction with thespectrometers 100 and 200 illustrated in FIGS. 1 and 2. As illustrated,the substrate 400 comprises a plurality of spatial regions A1-D4,arranged in a plurality of rows (identified by the letters A-D) and aplurality of columns (identified by the numbers 104). Although theillustrated substrate 400 comprises four rows and four columns for atotal of sixteen spatial regions, any number of spatial regions,arranged in any number of rows and/or columns may be deployed.

In one example, each of the spatial regions A1-D4 comprises a SERSsubstrate. For example, each of the spatial regions A1-D4 may include apatterned surface structure. This patterned surface structure may becreated in one example by a pattern of polymer fingers capped withmetallic nanoparticles. In one example, the patterned surface isdifferent for each of the spatial regions A1-D4. For instance, thegeometry of the polymer fingers may be varied from spatial region tospatial region by changing the distance between the polymer fingers, thespatial arrangement of the polymer fingers, the size of the polymerfingers, and/or other parameters. In addition, the functionalchemistries of the polymer fingers may be varied from spatial region tospatial region.

In the example illustrated in FIG. 4, for instance, each row (A-D) ofthe substrate 400 corresponds to a different spatial arrangement of thepolymer fingers (i.e., pentamer for row A, trimer for row B, dimer forrow C, and monomer for row D). In addition, each column (1-4) of thesubstrate 400 corresponds to a different spacing between the polymerfingers (as indicated by the arrows). By combining these parameters indifferent ways, sixteen unique spatial regions can be produced on thesubstrate 400.

In one example, the number of spatial regions on the substrate 400 isequal to the number of projection points in the two-dimensional patternthat is projected upon the substrate 400 (i.e., there is a one-to-onecorrespondence between the number of spatial regions and the number ofprojection points). The sampling apparatus 104 (or, more particularly,the objective lens 212) may project the two-dimensional pattern suchthat each projection point is incident upon a different one of thespatial regions.

In further examples, the functional chemistry of the patterned surfacestructure can be varied across the substrate 400. For instance, thefingers in one or more of the spatial regions A1-D4 may be fabricatedwith different surface functionalizations designed to interact withdifferent analytes, and thus provide different spectra.

FIG. 5 illustrates a flowchart of an example method 500 for moleculardetection. The method 500 may be performed, for example, by the examplespectrometer 100 illustrated in FIG. 1. As such, reference in made inthe discussion of the method 500 to various components of thespectrometer 100. However, the method 500 is not limited toimplementation with the spectrometer illustrated in FIG. 1.

The method 500 begins in block 502. In block 504, a beam of excitationlight is provided, for example by the light source 102. In one example,the beam of excitation light is a beam of visible, near infrared, ornear ultraviolet radiation.

In block 506, the beam of excitation light is converted into a pluralityof beams of excitation light traveling at different angles, for exampleby the holographic optical element 108. In one example, the single beamof excitation light may be “cleaned” to remove unwanted energy, such assecondary transmissions, background plasma, and other artifacts, priorto being converted into the plurality of beams of excitation light.

In block 508, the plurality of beams of excitation light is projected asa two-dimensional pattern of projection points onto a substrate, forexample by the objective lens 212. A sample whose molecular compositionis to be identified is adsorbed onto the substrate. Projection of thetwo-dimensional pattern may involve converting the different angles atwhich the plurality of beams of excitation light are traveling intodifferent positions or locations on the substrate. For instance, in oneexample, the substrate comprises an array of spatially varying regions,such as the substrate 400 illustrated in FIG. 4. In this case,projection of the two-dimensional pattern includes converting each angleat which one of the plurality of beams of excitation light is travelinginto a point location that is incident upon one of the spatial regionsof the substrate.

In block 510, emitted Raman scattering is collected from the substrate,for example by the objective lens 212. The emitted Raman scattering isemitted by the substrate in response to the incidence of the excitationlight of the two-dimensional pattern. In particular, the emitted Ramanscattering is produced by interactions of the molecules of the samplethat is housed upon the substrate with the excitation light. The emittedRaman scattering may be enhanced by the local surface topography of thesubstrate. When each region of the substrate has a different localtopography, the emitted Raman scattering may vary from region-to-region.In this case, multiple Raman spectra may be collected simultaneously inblock 510.

In block 512, the emitted Raman scattering is delivered to a detector,such as the detector 224. Subsequently, the detector may identify one ormore molecules contained in the sample, according to the signature(s) ofthe emitted Raman scattering. For example, by comparing a frequencyshift of the emitted Raman scattering to a database corresponding toreference measurements for a spatial region of the substrate from whichthe emitted Raman scattering was collected, one or more molecules of thesample may be identified. In one example, the emitted Raman scatteringis isolated from unwanted scattered energy, such as elastic scatteredradiation at the wavelength corresponding to the laser line (i.e.,Rayleigh scattering), prior to being delivered to the detector. Theemitted Raman scattering may also be dispersed, e.g., by the diffractiongrating 218, prior to being delivered to the detector.

The method 500 ends in block 514.

It should be noted that although not explicitly specified, some of theblocks, functions, or operations of the method 500 described above mayinclude storing, displaying and/or outputting for a particularapplication. In other words, any data, records, fields, and/orintermediate results discussed in the methods can be stored, displayed,and/or outputted to another device depending on the particularapplication. Furthermore, blocks, functions, or operations in FIG. 5that recite a determining operation, or involve a decision, do not implythat both branches of the determining operation are practiced. In otherwords, one of the branches of the determining operation may not beperformed, depending on the results of the determining operation.

Variants of the above-disclosed and other features and functions, oralternatives thereof, may be combined into many other different systemsor applications. Various presently unforeseen or unanticipatedalternatives, modifications, or variations therein may be subsequentlymade which are also intended to be encompassed by the following claims.

What is claimed is:
 1. An apparatus, comprising: a light source for emitting a beam of excitation light; a holographic optical element for converting the beam of excitation light into a plurality of beams of excitation light; a microscope for projecting the plurality of beams of excitation light onto a surface outside the apparatus as a two-dimensional pattern of projection points and for collecting scattered radiation emitted by the surface in response to the two-dimensional pattern of projection points; and a detector for detecting a frequency shift in the scattered radiation.
 2. The apparatus of claim 1, wherein the two-dimensional pattern of projection points comprises a rectangular array of projection points.
 3. The apparatus of claim 1, further comprising: a diffraction grating positioned between the microscope and the detector, for dispersing the scattered radiation prior to the scattered radiation being delivered to the detector.
 4. The apparatus of claim 1, wherein the surface outside the apparatus comprises: a surface-enhanced substrate.
 5. The apparatus of claim 4, wherein the surface-enhanced substrate comprises a plurality of regions, and a local topography of the substrate varies across the plurality of regions.
 6. The apparatus of claim 5, wherein the local topography comprises a pattern of polymer fingers, and at least some of the polymer fingers are capped with metal nanoparticles.
 7. The apparatus of claim 6, wherein variations in the local topography are created by varying a spatial arrangement of the polymer fingers across the plurality of regions.
 8. The apparatus of claim 6, wherein variations in the local topography are created by varying a spacing between the polymer fingers across the plurality of regions.
 9. The apparatus of claim 5, wherein the microscope is configured to arrange the two-dimensional pattern of projection points so that one of the projection points is incident upon each region of the plurality of regions.
 10. A method, comprising: providing a beam of excitation light; converting the beam of excitation light into a plurality of beams of excitation light; projecting the plurality of beams of excitation light onto a surface as a two-dimensional pattern of projection points, wherein each one of a plurality of different patterned structures scatters a different radiation pattern in response to an incidence of a respective beam of excitation light of the plurality of beams of excitation light; collecting scattered radiation emitted by the surface in response to the two-dimensional pattern of projection points, wherein the scattered radiation simultaneously includes multiple spectra of scattered radiation; and delivering the scattered radiation to a detector.
 11. The method of claim 10, further comprising: providing a surface-enhanced substrate onto which to project the two-dimensional pattern of projection points, wherein a sample containing at least one molecule is adsorbed onto the surface-enhanced substrate.
 12. The method of claim 11, wherein the providing comprises: providing a plurality of regions in the surface-enhanced substrate; and varying a local topography of the substrate across the plurality of regions.
 13. The method of claim 12, wherein the local topography comprises a pattern of polymer fingers, and at least some of the polymer fingers are capped with metal nanoparticles.
 14. The method of claim 13, wherein the varying comprises: varying a spatial arrangement of the polymer fingers across the plurality of regions.
 15. The method of claim 13, wherein the varying comprises: varying a spacing between the polymer fingers across the plurality of regions.
 16. The method of claim 12, wherein the projecting comprises: projecting one of the projection points upon each region of the plurality of regions.
 17. The method of claim 10, wherein the two-dimensional pattern of projection points comprises a rectangular array of projection points.
 18. An apparatus, comprising: a light source for emitting a beam of excitation light; a holographic optical element for converting the beam of excitation light into a plurality of beams of excitation light; a fiber optic probe for projecting the plurality of beams of excitation light onto a surface outside the apparatus as a two-dimensional pattern of projection points and for collecting scattered radiation emitted by the surface in response to the two-dimensional pattern of projection points; and a detector for detecting a frequency shift in the scattered radiation.
 19. The apparatus of claim 18, wherein the two-dimensional pattern of projection points comprises a rectangular array of projection points.
 20. The apparatus of claim 18, wherein the surface outside the apparatus comprises: a surface-enhanced substrate. 